Hospital admissions for flu rise as virus mutates

New Zealanders are catching the flu at three times the rate of last year, as the dominant strain of the virus begins to mutate.

Hospital emergency departments around the country saw a spike in admissions for influenza-like illnesses during July, with scientists hoping it was not related to a mutation in the Influenza A virus. Auckland City Hospital said it had had the ‘‘busiest July ever’’ this year, with 136 influenza hospital admissions – up from just 41 in July last year.

Wellington has also seen more admissions in July, with 96 patients with influenza-like symptoms treated in Wellington Hospital alone.

Institute of Environmental Science and Research public health physician Jill Sherwood said rates of influenza were three times higher than last July.

This year it was seeing 50 cases per 100,000 people, compared to 17 at the same time in 2016.

Sherwood said the main strain of Influenza A, H3N2, was beginning to show signs of mutation.

It was too early to say if this was affecting the efficacy of the vaccine, but scientists would closely monitor the strain for the rest of the flu season.

New Zealand’s 2017 vaccine covered for two strains of Influenza A and one strain of Influenza B, called Victoria.

But the main strain of B virus this year was Yamagata, which had been spreading through the population.

Sherwood said people who had been vaccinated would have some crossprotection from this virus.

In 2016, the national laboratory identified 24 cases of Yamagata for the entire flu season, from May to September.

This year, there had already been 260 confirmed cases.

Higher influenza rates had been predicted this year, as 2016 had been an ‘‘unusually mild’’ season – and the Northern Hemisphere had also seen an increase during its winter, Sherwood said. Rates were still considered seasonally low, and were about half that of 2015.

‘‘The way the flu season is tracking, it looks like yes, we are seeing some increases, but nothing dramatic.’’

National Influenza Immunisation Awareness campaign spokesman and virologist Lance Jennings said the influenza vaccination was still the single best protector against getting sick.

The vaccine is usually 50-60 per cent effective, and is reassessed next year.

Medsafe and the Ministry of Health are considering a new quadrivalent vaccine for 2018, which would include the B/yamagata strain. Since May 1, the start of the flu season, samples from 670 people tested positive for influenza in the ESR laboratory. Last year, there were 294 confirmed cases over the entire flu season, from May to September.